1. Field of the Invention
The present invention relates generally to mechanical presses for processing workpieces such as metals or other materials and more particularly to a method and devices which facilitates safe operation of mechanical presses.
2. Description of the Prior Art
In mechanical presses, a ram holding an upper tool or die is vertically moved in processing operations by means of a crank shaft or eccentric shaft toward and away from a lower tool or die to process workpieces which are placed on the lower die. For manual operations in which workpieces are manually fed onto and removed from the lower die by hand, mechanical presses have to be so designed that the ram may be positively stopped at its uppermost limit of travel after completion of each working cycle or stroke. As is well known, mechanical presses are provided with a clutch and a brake which are often provided as an assembly and are mostly pneumatically operated, and the ram is stopped when the clutch and the brake are simultaneously operated. Of course, the clutch is so designed as to connect and disconnect the crank shaft for driving the ram with the power source such as a flywheel and a motor, while the brake is operated to stop the crank shaft simultaneously when the clutch disconnects the same from the power source.
In mechanical presses, it has been that the ram often fails to stop at its uppermost travelling limit after completing it stroke and will repeat another stroke because of malfunction of the clutch and the brake or electric or pneumatic means for controlling these elements, e.g., a solenoid operated valve, or for any other reasons. Needless to say, there is a danger of injuring the operator of the press if the ram repeats an unintended stroke without stopping at its uppermost travelling limit. Actually, accidents in operations with mechanical presses have happened mostly from such unintentional repeated strokes of the rams.
Although some mechanical presses are doubly equipped with solenoid operated valves for controlling the clutch and the brake for extra safety, of course this arrangement could not prevent the ram from unintentionally repeating its stroke owing to malfunctions other than that of the solenoid operated valve. Also, some presses are so constructed that the crank shaft is mechanically and forcibly stopped by a stop means such as a pin from unintentionally rotating past its upper dead center to stop the ram from double stroking past its uppermost travelling limit. However, this arrangement not only could not positively prevent the ram from double stroking except when the clutch is incompletely connecting the crank shaft with the power source, but also it tends to damage the press and its components because of the shock occuring when the ram is forcibly stopped.